Self Propelled Artillery: M109 replacements

[DWG]

That little gun on top of the model is supposed to easily take out tanks?

Research on RLGLP possible velocities would answer your question.

And yet no one is proposing replacing 120mm guns with 25mm. Penetration is roughly related to calibre, which means a 25mm would need about five times the performance of a 120mm assuming a linear relationship and scaling the round design (25x if it's a square relationship - can't check right now). If instead you assume a non-saboted 25mm long rod penetrator, boosted to the same MV as a saboted long rod from a 120mm, then you end up with an extremely unwieldy breech and feed system to cater for the c0.8m length of the bare long rod.

 

[DWG]

A 25mm RGLP would easily been able to defeat tanks threatening the M-109 allowing it to operate much farther forward.

Uu
The 25mm being proposed for the FCS M109 replacement concepts was the low velocity OCSW in the counter-infantry/light vehicle/ATGM suppression role, not for defence vs tanks. The reference here to a 25mm LP weapon is almost certainly looking at the same role.

 

[Elan Vital]

I didn't know where best to post this since we don't have a thread related to M109 prototypes and wasn't sure if one would be useful, but here are engineering reports for respectively the Low Heat Rejection version of the 8V71T engine for the M109A6 that made it to the baseline vehicle, and a 500hp version tested in 2003 which failed due to limitations in the cooling system. There were very interesting points on how cooling could be improved eventually.

LHR engine is added as a file, other is too big so I will refer to the DTIC doc: ADA445815